In wavelength division multiplexed optical communication systems, many different optical wavelength carriers provide independent communication channels in a single optical fiber. Future computation and communication systems place ever-increasing demands upon communication link bandwidth. It is generally known that optical fibers offer much higher bandwidth than conventional coaxial communications; furthermore a single optical channel in a fiber waveguide uses a microscopically small fraction of the available bandwidth of the fiber (typically a few GHz out of several tens of THz). By transmitting several channels at different optical wavelengths into one fiber (i.e., wavelength division multiplexing, or WDM), this bandwidth may be more efficiently utilized.
There have been many attempts to develop a compact, high-resolution waveguide demultiplexor or spectrometer for application in areas such as spectroscopy, optical networks and optical links and more particularly optical communication systems. Such a demultiplexor can be extremely critical in wavelength division multiplexing (WDM) links. In these links or networks, each channel is assigned a distinct and unique wavelength for data transmission. Thus, the optical fiber that connects channels in a WDM network carries many discrete wavelength channels and a particular wavelength is selected before the data is received. The data reception can be achieved by combining a wavelength demultiplexor, photodetectors and electronic selection circuitries. In WDM links, many wavelengths are multiplexed and transmitted through a single optical fiber to increase the capacity of the fiber. The receiver must demultiplex the many wavelengths and select the proper channel for reception. In these applications, the requirements on the wavelength demultiplexor are typically: an optical bandwidth&gt;30 nm, a wavelength resolution of a few angstroms, polarization insensitivity, compactness, low loss, low crosstalk, and a low manufacturing cost.
At present, there are many known methods of selecting particular wavelengths, however, none are ideal for the applications outlined above.
Techniques for multiplexing and demultiplexing between a single optical fiber comprising the multiplexed channel and plural optical fibers comprising the plural demultiplexed channels are described in various U.S. patents. For example, multiplexing/demultiplexing with birefringent elements is disclosed in U.S. Pat. Nos. 4,744,075 and 4,745,991. Multiplexing/demultiplexing using optical bandpass filters (such as a resonant cavity) is disclosed in U.S. Pat. Nos. 4,707,064 and 5,111,519. Multiplexing/demultiplexing with interference filters is disclosed in U.S. Pat. Nos. 4,474,424 and 4,630,255 and 4,735,478. Multiplexing/demultiplexing using a prism is disclosed in U.S. Pat. No. 4,335,933. U.S. Pat. No. 4,740,951 teaches a complex sequence of cascaded gratings to demultiplex plural optical signals. U.S. Pat. Nos. 4,756,587 and 4,989,937 and 4,690,489 disclose optical coupling between adjacent waveguides to achieve a demultiplexing function. A similar technique is disclosed in U.S. Pat. No. 4,900,118. Although some of these techniques are better than others, there is a need for a system that is compact and which does not rely on bulk grating elements that is relatively inexpensive to manufacture and that is provides reasonable precision.
Optical switching, multiplexing and demultiplexing has been accomplished for nearly a decade by using an interconection apparatus having a plurality of closely spaced waveguides communicating with an input star coupler. The output of the star coupler communicates with a second star coupler via an optical grating consisting of an array of optical waveguides. Each of the waveguides differs in length with respect to its nearest neighbour by a predetermined fixed amount. The ouputs of the second star coupler form the outputs of the switching, multiplexing and demultiplexing apparatus. See for example U.S. Pat. No. 5,002,350 in the name of Dragone, issued Mar. 25, 1991.
In operation when each of a plurality of separate and distinct wavelengths are launched into a separate and distinct input port of the apparatus, they will all combine and appear on a predetermined one of the output ports. In this manner, the apparatus performs a multiplexing function. The same apparatus may also perform a demultiplexing function. In this situation a plurality of input wavelengths is directed to a predetermined one of the input ports of the apparatus. Each of the input wavelengths is separated from the other and directed to a predetermined one of the output ports of the apparatus. An appropriate selection of the input wavelength also permits switching between any selected input port to any selected output port.
The grating located between the two star couplers essentially consists of an array of curved waveguides of different lengths. The waveguides are closely spaced at their ends, whereas they are widely spaced and strongly curved in the central region. The order of the grating is determined by the difference in length between the adjacent waveguides. U.S. Pat. No. 5,243,672 also in the name of Dragone, issued Sep. 7, 1993 describes an improved method of making such a grating with an improved bend radius. Cohen et al. in U.S. Pat. No. 5,440,416 describes a similar grating wherein reflection is utilized and the structure is cut in half.
Although each of these patents has it merits and describes working devices, the performance of all of these devices is susceptible to temperature variations. One standard means for stabilizing the output of such devices is to actively control the temperature about these devices. Thus, control circuits with heating elements are provided to ensure a stable temperature environment. Of course, there are limits to such control; and furthermore, these devices generally consume considerable power.
Currently these phase array wavelength division multiplexors are fabricated on a monolithic glass slab, wherein waveguides and a transformation region are disposed means integrated therein for separating an input beam into sub-beams having different central wavelengths; said second portion having closely spaced waveguides for receiving the sub-beams of light and having passive temperature compensation means coupled therewith for enhancing the coupling of the optical signals having predetermined wavelengths provided by the first block with the closely spaced waveguides.
It is an advantage of the present invention that optical coupling of light within a free space region of a phased array demultiplexer to one or more waveguides can be enhanced without significant energy requirements for temperature control. It is a further advantage that coupling can be enhanced using a passive system requiring no additional energy.
Further advantages will be apparent to persons of skill in the art with reference to the following description of preferred embodiments, and to the drawings.